The present invention relates in general to an arrangement for the transmission of rotary motion into a vacuum chamber, and in particular to a new and useful apparatus which has a shaft held in a bore of the chamber wall for transmitting the rotary motion, as well as sealing rings between the inner wall of the bore and the shaft for creating a vacuum-tight seal.
In rotary transmission lead-throughs with seals of conventional design (for example, lip seals), lubrication of the sliding surfaces is absolutely necessary. The lubricant has primarily the function of forming a lubricating film between the sliding surfaces of the shaft and the sealing rings, and the reciprocal pressure with which the sliding surfaces press against each other, must always be dimensioned so that a sufficiently thick lubricating film can form. In the most favorable case, a coefficient of friction of approximately 0.05 can be achieved. Such lubricating film, however, always has the undesirable property of flowing along the surface wetted by it, and in this way, it can also reach the interior of the vacuum chamber and contaminate it. If the attempt is made to decrease the supply of the lubricating agent, for example, by increasing the pressure between shaft and sealing rings, or by using highly viscous lubricating oils or greases, this not only leads to an undesirable increase of the coefficient of friction, but also often to the fact that the sliding surfaces run dry due to the poor emergency running properties of the sealing material, often to the so-called "eating away" (pitting) point which represents non-repairable damage and, consequently, improper sealing. To this day, no sealing material has been found which has sufficiently reliable emergency running properties under moderately strong load relative to metallic surfaces. By using molybdenum disulfide as a slide agent, the coefficient of friction, even when running dry, can be kept relatively low, however, this does not solve the problem. Such lubricating film, once damaged, does not repair itself, but the damage can progress and can eventually bring about total loss of the emergency running properties. In addition, the coefficient of friction (at approximately 0.1=.mu.) is still relatively high, so that under great load, and due to the poor ability to conduct heat of all sealing materials, thermal over-stressing of the seal can occur.